1. Field of the Invention
The present invention relates to a reactive power compensator for compensating reactive power by injecting a reactive current into an alternating-current power system.
2. Background Art
In an alternating-current power transmitting system using a power transmission line or a power transmission cable, reactive power is increased and the alternating voltage tends to be changed when power transmission distance is increased or the electric power transmitted is increased due to the influence of impedance and earth capacitance of the power transmission line or the power transmission cable. This change of the alternating voltage becomes a factor for destabilizing the operation of the power transmission system. Various kinds of reactive power compensators for injecting reactive current into the alternating-current power system are used to solve this destabilization of this alternating-current power transmitting system.
As this reactive power compensator, a type of the reactive power compensator for converting the direct current voltage of a direct current power source circuit into an alternating voltage by using an electric power converting circuit of an electric current control type using a switching element and supplying this alternating voltage to the alternating-current power system is proposed in e.g., JP-A-2000-83386. Since the reactive power compensator of this type has high control performance, the reactive power compensator of this type can also restrain the change of a transient alternating voltage based on the change of the reactive power induced when an accident is occurred in the alternating-current power system as well as stationary reactive-power compensation. Accordingly, the reactive power compensator of this type can contribute to the stabilization of the alternating-current power system.
The reactive power compensator shown in FIG. 1 of JP-A-2000-83386 has a control command means for outputting an alternating voltage command signal with respect to the electric power converting circuit, and an alternating voltage control means for controlling the operation of a switching element of the above electric power converting circuit in accordance with the above alternating voltage command signal. The above control command means generates an electric current detecting signal by an alternating voltage detecting signal showing the alternating voltage in the alternating-current power system, and an alternating electric current detecting signal showing the alternating electric current in the alternating-current power system. The above control command means also generates an alternating voltage command signal by comparing this electric current detecting signal with an electric current command signal. This alternating voltage command signal is given to the alternating voltage control means. This alternating voltage control means controls on-off timing of the switching element of the electric power converting circuit, and controls the above electric current detecting signal so as to be conformed to the above electric current command signal.
[Patent literature 1] JP-A-2000-83386, particularly, FIG. 1 and its explanation
In the reactive power compensator shown in the above related art, ability able to conform the above electric current detecting signal to the above electric current command signal by the above control command means depends on a control loop gain of a control circuit including the above control command means and the above alternating voltage control means, and the number of control times of the on-off timing of the switching element of the above electric power converting circuit. As the number of control times of the on-off timing of the above switching element is increased, the deviation between the above electric current detecting signal and the above electric current command signal can be reduced. Further, as the control loop gain of the above control circuit is raised, the deviation between the above electric current detecting signal and the above electric current command signal can be reduced in a wider frequency range.
However, when the number of control times of the on-off timing of the above switching element is increased, electric power loss in the switching element is increased and efficiency of the electric power converting circuit is reduced so that economical loss is increased. The number of control times of the on-off timing of the switching element is limited and operated to restrain the increase of this loss such that response characteristics of the above control circuit become a required minimum value.
On the other hand, resonance depending on a higher harmonic wave vibration from a higher harmonic wave generating source and frequency characteristics of impedance exists in the alternating-current power system. Accordingly, a higher harmonic wave voltage based on the higher harmonic wave vibration and the resonance is also applied to the electric power converting circuit, and a higher harmonic wave electric current based on these is flowed. When the number of control times of the on-off timing of the above switching element is increased, the deviation between the above electric current detecting signal and the above electric current command signal can be also reduced with respect to the higher harmonic wave voltage and the higher harmonic wave electric current. However, as mentioned above, when the number of control times of the on-off timing of the switching element is limited to restrain the increase of the loss, it is impossible to cope with the higher harmonic wave voltage and the higher harmonic wave electric current. There is also a danger that an excessive higher harmonic wave electric current is flowed to the electric power converting circuit. When the electric current flowed to the switching element exceeds its controllable range due to this excessive higher harmonic wave electric current, a protecting operation for stopping the on-off control of the switching element is performed to prevent damage of the device and a reactive power compensating function is also stopped.
Thus, in the conventional reactive power compensator shown in the above related art, a problem exists in that the electric current flowed to the switching element exceeds its controllable range and the reactive power compensating function is stopped due to the higher harmonic wave voltage and the higher harmonic wave electric current of the alternating-current power system.